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Hi All

When we used an AOM for 2P intensity modulation we used the zero order beam and with 95% diffraction efficiency the 2P excitation rate was so low when the AOM was fully energized that the zero order beam was effectively shut off. For safety there was also a 2ms response time mechanical shutter that would block the beam when not scanning. That  said, we did not expect the AOM to provide full beam intensity control, but had a separate attenuator (reflective ND filter) to set the maximum power desired.

HTH Mark

Mark Cannell Ph.D. FRSNZ, FISHR
University of Bristol, UK

On 9/26/19, 7:12 AM, "Confocal Microscopy List on behalf of Peter Müller" <[log in to unmask] on behalf of [log in to unmask]> wrote:

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    Sudipta, Ben,
    
    thanks for your really nice answers.
    
    We do want fast modulation (>1MHz).
    
    In fact we have seen some spatial chirp from the AOM, mostly in form of an slightly elliptical beam profile. The blue wavelength components of the laser get less diffracted than the red components and will spatially separate, as you mention. This gets worse with an increased AOM carrier frequency (typ. 80MHz). But if go through the math, you'll notice that you need more than 5 meters of free space propagation for a 100 fs beam at 800nm to significantly separate the beam's components.
    
    Best,
    Peter
        Am Dienstag, 24. September 2019, 19:57:18 MESZ hat maiti <[log in to unmask]> Folgendes geschrieben:  
     
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    Dear Peter,
    
    I concur with Ben Smith's beautifully detailed reply. I think a half waveplate and 
    polarizer combo is the best and the simplest option for intensity control, and we 
    use it regularly. However, even if you have the half waveplate mounted on a 
    stepper motor, it is not going to be a fast modulation option. 
    
    I have other worries about AOM's, in addition to the GVD. Since they are 
    diffractive elements, they would introduce spatial chirp if used with a 
    femtosecond beam with a large spectral content ( if we are not using the zero 
    order). However, I have not properly tested them to see if this  really matters. 
    May be the spectral chirp is properly compensated by the AOM manufacturers. Has 
    anyone in this group tested it for the spatial chirp?
    
    Sudipta
    
     
      On Tue, 24 Sep 2019 06:03:38 +0000, Peter Müller wrote
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    > 
    > Dear Microscopists,
    > for our multiphoton scope we use an acousto-optical modulator for 
    > intensity modulation. Those AOM have a nice overall throughput (>85%)
    >  and a high bandwidth (>1MHz), however they add a lot of group delay 
    > dispersion (GDD) to our setup - typically they use a 1cm - 2cm TeO2 
    > crystal for the NIR wavelengths from 700nm to 1000nm. For our 
    > multiphoton setup, the AOM is the main contributor of GDD right now. 
    > I was wondering: what is your preferred choice for intensity 
    > modulation and what are the respective advantages/disadvantages?
    > 
    > Thanks,Peter
    
    
    Prof. Sudipta Maiti
    Dept. of Chemical Sciences 
    Tata Institute of Fundamental Research
    Homi Bhabha Road, Colaba
    Mumbai 400005, India
    Ph. +91 222 278 2716 
    Alternate e-mail: [log in to unmask]
    webpage: biophotonics.co.in